Subminiature circuit protectors are useful in applications in which size and space limitations are important, for example, on circuit boards for electronic equipment, for denser packing and miniaturization of electronic circuits.
Ceramic chip type fuses are typically manufactured by depositing layers of metal elements on a ceramic or glass substrate plate, attaching an insulating cover over the deposited layers, and cutting, or dicing, individual fuses from the finished structure. The Cutting operation is difficult and expensive to carry out. In addition, subminiature fuses made with deposited film fuse elements are generally limited to low voltage and current interrupting capacity.
The present invention, generally, provides a method of manufacturing a subminiature surface mountable circuit protector that is simple and relatively inexpensive. The present invention also provides a subminiature circuit protector that has improved short circuit current interrupting capacity compared to conventional circuit protectors of similar physical size.
More particularly, the present invention provides a method of manufacturing a multiplicity of subminiature circuit protectors from a plate of substrate material that facilitates the formation and rapid cutting of the substrate into individual units.
According to the present invention, a substrate plate of green, or unfired, ceramic material is prepared. Electrically conductive metallic film is deposited on a top surface of the substrate plate in equally spaced, parallel columns. Fuse elements, in the form of electrically conductive wires, are disposed on the top surface of the substrate perpendicular to the film columns, in equally spaced parallel rows. A second plate of green ceramic material is laminated to the substrate over the film columns and wire elements rows. The second plate covers and encapsulates the film columns and wire rows. The thus formed structure is then die cut, that is, cut, longitudinally through the metal film columns and transversely between the wire rows so that individual units are produced having strips of metal film at opposite ends and a wire element extending from end to end across a space between the metal film strips. The die cut individual units are fired to cure the ceramic substrate and cover plate and to cause an intermetallic bond to form between the wire elements and the metal film. The ends of the individual units are coated with electrically conductive materials to form electrical terminations for connecting in a circuit.
According to one aspect of the invention, the wire elements may be applied to the substrate by rolling and pressing the wire into the substrate. The application of pressure imbeds the wire elements in the substrate and helps form contact between the wire elements and the metallic film.
According to another aspect of the invention, the laminate structure is die cut so that the individual units formed have opposite ends faces and opposite lateral faces. A metal strip at each opposite end of each unit extends to the end face and to both lateral faces so that the electrical termination coatings applied to the units contact the metal strips on the end and lateral faces.
According to yet another aspect of the invention, the end termination coatings comprise a first coating of silver or a silver alloy. A second coating of nickel is applied over the first coating. A third coating of a tin/lead alloy is applied over the nickel coating.